Evidence for additional structure in the effective spin distribution hints at multiple formation pathways in GWTC-5.0

Abstract

The distribution of the effective inspiral spin ($χ_\mathrm{eff}$) of the binary black holes detected by LIGO-Virgo-KAGRA can shed light on their formation pathways. We analyze the GWTC-5.0 dataset with two models-one flexible, one fully parametric-that jointly describe $χ_\mathrm{eff}$ and primary mass. We clarify that the previously-reported skewness in the $χ_\mathrm{eff}$ distribution is better understood as additional structure beyond a non-skewed Gaussian bulk centered at small $χ_\mathrm{eff}$. This additional structure extends to larger $|χ_\mathrm{eff}|$, a result previously reported using GWTC-4.0 data. We measure the asymmetry of the distribution of $χ_\mathrm{eff}$ outside the Gaussian bulk from the data. With both the parametric and the flexible analyses, we find suggestive evidence for a mass-dependent excess of positive $χ_\mathrm{eff}$ over negative ones outside the Gaussian bulk. We find only a mild statistical preference for a negative $χ_\mathrm{eff}$ component outside the Gaussian bulk. This preference is highest at $m_1 \in [47,65]\,M_\odot$, with $13\text{:}1$ odds. Excluding the event GW191109, whose data have known data-quality issues, reduces these odds to $5\text{:}1$. If $χ_\mathrm{eff}$ outside the Gaussian bulk are produced by hierarchical mergers-as has been suggested-then a fraction of those mergers may be produced in environments that can generate a surplus of binaries with positive $χ_\mathrm{eff}$, such as the disks of active galactic nuclei.